Gas-surface interfacial tribochemistry and superlubric mechanism of a-C:H:Si films in different gaseous atmospheres

摘要

The term "superlubricity" describes a theoretical sliding state in which friction or resistance to the sliding completely vanishes, on the basis of an incommensurability theory originally proposed by Hirano [1]. Recently, the existence of such a phenomenon was also discovered on a range of hydrogen-rich a-C:H:Si films by providing friction coefficient lower than 0.01 both in humid air and inert gas atmospheres [2-4]. Overall, the most important requirement for achieving superlubricity in a-C:H:Si films is a optimum combination of intrinsic (film structure) and extrinsic (test environments) factors. Intrinsically, the silicon content controls the OH-passivation originated from adsorbed water dissociation in humid air while the hydrogen (bonded and unbonded hydrogen molecules) incorporation as well as sp~2-C structure determines the repulsive H-termination and interfacial shearing behavior in inert atmosphere, respectively. In addition, atomically smooth surface (e.g., Ra=0.1 nm) is also desirable to shorten the running-in period during contact process. Extrinsically, the presence or absence of certain chemical species in the gaseous tribotesting environment plays a prominent role in affecting the gas-surface interfacial tribochemistry and frictional behaviors. In this work, we choose a polymer-like a-C:H:Si film to be further tribotested in various gaseous atmospheres including humid air, inert gases (N_2 and Ar), reactive gas (H_2) and corrosive gas (O_2). Some striking findings concerning the tribochemistry and interfacial structural evolution depending on the gas-surface interactions during sliding are shown to disclose the underlying superlubric mechanism of a-C:H:Si films.